Hybrid vehicles include an engine and an electric machine that operates as a motor/generator with an associated battery to provide an electric vehicle (EV) mode using only electric power to propel the vehicle, or a hybrid electric vehicle (HEV) mode that uses the engine and electric machine to propel the vehicle. Some hybrid vehicle powertrain configurations include a disconnect clutch configured to selectively disengage the engine from the electric machine and transmission while operating in EV mode.
While operating in EV mode, hybrid vehicles may start the engine (engine pull up) using a starter motor. Alternatively, the engine may be started by engaging (or closing) the disconnect clutch and using the electric machine to start the engine. During disconnect clutch engagement, a torque converter bypass clutch may be opened to isolate the driveline from torque disturbance. Opening and/or slipping the torque converter bypass clutch acts like a low pass filter and dampens the torque disturbances from propagating through the driveline.
Engine pull-up requests depend on multiple considerations such as accelerator tip in/out, total driver power demand and maximum available motor torque. Many engine pull-up strategies only account for maximum available motor torque at present or current impeller speed and do not account for the change in maximum available motor torque due to increase in motor speed as a result of opening the torque converter bypass clutch during engine starts. If the engine start/engine pull-up threshold does not account for this dynamic change in available motor torque, opening/slipping of torque converter bypass clutch during engine starts may result in a torque hole.